1. Field of the Invention
The present invention relates to an inkjet recording head for recording characters and images by ejection of ink droplets, and manufacturing and driving methods thereof, and an inkjet recording apparatus having such an inkjet recording head.
2. Description of the Related Art
In recent years, an impact recording process has attracted much attention for its small noise in recording and a high recording speed thereof. Among other impact recording processes, an inkjet recording process used in inkjet printers has been in wide use. The inkjet printer allows ink droplets to be ejected from the recording head and attached onto recording paper so that characters, figures and photographs are printed at a high speed. The inkjet printer is capable of recording the images onto plain paper without using a special fixation processing. According to a known inkjet recording process called drop-on-demand inkjet recording scheme, an electromechanical transducer such as a piezoelectric actuator is used to generate pressure waves (acoustic waves) in pressure chambers filled with ink, thereby allowing ink droplets to be ejected from the nozzles disposed in communication with the pressure chambers.
An inkjet recording head using the drop-on-demand inkjet scheme is described in JP Patent Publication No. Sho. 53-12138 and JP Patent Laid-Open Publication No. Hei. 10-193587. FIG. 34 is a sectional view of a recording head in an inkjet recording apparatus such as described in these publications. The inkjet recording apparatus includes a plurality of pressure chambers 51, a plurality of nozzles 52 each in communication with a corresponding one of the pressure chambers 51, a plurality of ink supply passage 54 for supplying ink from an ink reservoir through a common ink passage 53. A diaphragm 55 is fixed onto the bottom of the pressure chambers 51.
In the above inkjet recording apparatus, at the time of ink droplet ejection, the diaphragm 55 is displaced (or flexibly deformed) by a piezoelectric actuator 56 disposed outside the pressure chamber 51 to change the volume in the pressure chamber 51, thereby generating a pressure wave in the pressure chamber 51. The pressure wave causes part of the ink filled in the pressure chamber 51 to be ejected outside the pressure chamber 51 through the nozzle 52 as an ink droplet 57. The ejected ink droplets arrive at a recording medium such as a recording paper sheet to form recording dots (pixels) thereon. The process of forming the recording dots is iteratively performed based on image data so that characters or images are formed on the recording medium.
It is desired that the drop-on-demand inkjet recording apparatus achieve both high speed recording and high image quality recording. In the conventional inkjet recording apparatus, however, achieving both high speed recording and high image quality recording at the same time is in fact extremely difficult. If, for example, the resolution is degraded in order to achieve high speed recording, high image quality is not provided. On the other hand, if the resolution is increased in order to achieve high image quality recording, the high speed recording cannot be obtained. In other words, high speed recording and high image quality recording are tradeoff against each other.
The necessary conditions to achieve both xe2x80x9chigh speed recordingxe2x80x9d and xe2x80x9chigh image quality recordingxe2x80x9d in the inkjet recording apparatus will be described hereinafter. Two particularly important conditions to achieve the xe2x80x9chigh speed recordingxe2x80x9d are as follows:
(1) to lower the recording resolution, and
(2) to increase the number of nozzles disposed (or to increase the nozzle density).
If the condition (1), i.e., to lower the resolution is satisfied, a unit area can be recorded with less ink droplets, and therefore the time required for recording can be reduced. When recording resolutions of 300 dpi (dots/inch) and 1200 dpi are compared against each other, the necessary number of ink dots for 300 dpi is {fraction (1/16)} as many as that for the resolution of 1200 dpi to record the same area. It should be noted that, for the same frequency of ejected ink droplets (driving frequency), the recording speed obtained at 300 dpi resolution is about 16 times as high as that obtained at 1200 dpi resolution.
However, if the recording resolution is set to be lower, the image quality is lowered, and therefore there is a lower limit for the reduction in the recording resolution. In consideration of human eyesight, it is most suitable that the recording resolution be set in the range roughly from 300 to 600 dpi (1 dot/inch =39.37 dots/m) in order to achieve high speed recording without significantly lowering the image quality (character and line qualities). The recording resolution is preferably set to be lower than the usual resolution (700 to 2400 dpi) of typical inkjet recording apparatuses generally used heretofore, in view of improvement of the recording speed. It should be noted, however, that in order to set a lower recording resolution, ink droplets having a corresponding large size should be ejected.
More specifically, in order to form large ink dots in accordance with the high speed recording corresponding to the lower recording resolution, ink droplets having a corresponding large volume should be ejected. The relation between the recording resolution and a necessary droplet volume changes to some extent depending on the types of ink or recording paper used. With a typical ink and typical recording paper used in the conventional inkjet recording apparatus, an ink droplet volume in the range from 15 to 30 pl (1 pico-liter=10xe2x88x9215 m3) is generally adopted to provide a sufficient recording density together with a recording resolution in the range from 300 to 600 dpi. This is about 1.5 to 3 times as much as the ink droplet volume (about 10 pl) necessary for the resolution of 1200 dpi.
In order to increase the recording speed, the number of nozzles should be increased as in the above condition (2). A larger number of nozzles increases the number of dots formed per unit time length, which improves the recording speed. Therefore, in typical inkjet recording apparatuses, a multi-nozzle recording head is generally employed which includes a number of such ink ejecting mechanisms (ejectors) coupled together.
FIG. 35 shows the multi-nozzle recording head. In the recording head shown, an ink reservoir 67 is coupled with a common ink passage 63, to which a plurality of pressure chambers 61 are coupled through respective ink supply passages (not shown). Thus, in this head structure, by arranging ejectors 68 one-dimensionally along the common ink passage 63, the number of ejectors 68 (or the number of nozzles 62) is increased up to about 30 to 100.
Another inkjet recording head having ejectors arranged two-dimensionally in a matrix (hereinafter referred to as xe2x80x9cmatrix headxe2x80x9d) to further increase the number of ejectors is described in, for example, JP Patent Laid-Open Publication No. 1-208146 and JP National Phase PCT Publication No. 10-508808. FIG. 36 shows a matrix head such as described in these publications. In the matrix head, the common ink passage includes a main passage 73 and a plurality of branch passages 78, wherein a plurality of ejectors 79 are connected to a single branch passage 78. The matrix head structure is advantageous in increasing the number of ejectors 89 (or the number of nozzles 72). If, for example, there are 26 branch passages 78 disposed in the matrix head, and ten ejectors 79 are connected to each of the branch passages 78, then 260 ejectors can be arranged altogether. It should be noted that FIG. 36 shows only 36 ejectors among them.
As described above, the matrix head is advantageous for increasing the number of nozzles, while the dimensions of the recording head as a whole increases unless the density of the pressure chambers 71 arranged is not increased. This may increase the cost for manufacturing the recording head and the size of the recording apparatus, and increases the distance to transport the recording head, which lowers the recording speed.
More specifically, increase in the number of nozzles in the inkjet recording head means to arrange more nozzles in a specified area. In other words, the object can be interpreted as how to increase the nozzle density. In the matrix head as shown in FIG. 36, the dimensions of the pressure chamber 71 should be reduced in order to increase the density of the ejectors 79 arranged.
On the other hand, in order to achieve the xe2x80x9chigh image quality recordingxe2x80x9d in the inkjet recording apparatus, the size of ejected ink droplets is preferably reduced as much as possible. When a photographic image is output, in particular, the perceived granularity of a highlight part (lower density part) significantly affects the image quality, and therefore the highlight part is preferably recorded with extremely small ink droplets. Because of human eye resolution, the granularity perceived by a human eye significantly reduces at a dot size of 40 micrometers or less. At 30 micrometers or less, individual dots can no longer be perceived by the human eye as such, and therefore the image quality remarkably improves. As a result, the highlight part of an image preferably has dots having a diameter of 30 micrometers or less. Thus, very small ink droplets having a volume of about 2 to 4 pl should be preferably ejected.
A method of driving the inkjet recording head to eject very small droplets is described in, for example, JP Patent Laid-Open Publication No. 55-17589. According to the described driving technique, the pressure chamber is expanded immediately before ink ejection, and then an ink droplet is ejected from the nozzle in the state wherein the meniscus in the nozzle orifice is pulled toward the pressure chamber side. The driving waveform such as used in this driving technique is shown in FIG. 37. The driving waveform includes a first section or a first voltage change process 83 by which the pressure chamber is expanded, and a second section or a second voltage change process 84 by which the pressure chamber is compressed to eject the ink droplet.
FIGS. 38A to 38D are schematic sectional views for illustration of the movement of a meniscus 92 at the orifice of the nozzle 91 when a voltage having the driving waveform shown in FIG. 37 is applied. As shown in FIG. 38A, the surface of the meniscus 92 is flat in the initial state. After the pressure chamber starts to expand in response to the first voltage change section 83 shown in FIG. 37, the center of the meniscus 92 is retracted greatly from the peripheral part, so that the meniscus 92 has a concave shape as shown in FIG. 38B.
In response to the second voltage change section 84 shown in FIG. 37, after the state in which the concave meniscus 92 is formed, the pressure chamber starts to be compressed, and a small-diameter liquid column 93 is formed in the center of the meniscus 92 as shown in FIG. 38C. Then, as shown in FIG. 38D, the tip portion of the liquid column 93 is separated from the rest of the liquid column 93 to form an ink droplet 94. The size of the ink droplet 94 is substantially equal to the diameter of the liquid column 93, and thus smaller than the nozzle diameter. In short, an ink droplet 94 having a size smaller than the nozzle diameter can be ejected by this driving technique. In this text, the driving technique that controls the meniscus shape and ejects very small droplets, as described above, will be referred to as xe2x80x9cmeniscus control technique.xe2x80x9d
As described above, in order to achieve the xe2x80x9chigh speed recordingxe2x80x9d using the drop-on-demand inkjet recording head, large droplets should be ejected to achieve low resolution recording and the nozzle density should be increased to increase the number of nozzles disposed per unit area. On the other hand, in order to achieve high image quality recording, small droplets should be ejected to reduce the granularity of the highlight part. In order to achieve both xe2x80x9chigh speed recordingxe2x80x9d and xe2x80x9chigh image quality recordingxe2x80x9d using a single recording head, the three conditions, i.e., xe2x80x9cejection of large droplets,xe2x80x9d xe2x80x9cincrease in the nozzle density,xe2x80x9d and xe2x80x9cejection of small dropletsxe2x80x9d should be satisfied.
However, all the three objects i.e., xe2x80x9cejection of large dropletsxe2x80x9d and xe2x80x9cincrease in the nozzle densityxe2x80x9d for high speed recording and xe2x80x9cejection of small dropletsxe2x80x9d for high image quality recording can hardly be satisfied at a time using the conventional inkjet recording head.
Another disadvantage associated with the conventional inkjet recording head is abnormal vibration of the meniscus at the time of ink droplet ejection and instability caused by the vibration in the ejection of ink droplets. There had never been a detailed study disclosing the mechanism of how such abnormal meniscus vibration was caused, or of a method of preventing such a vibration. Now, the result of study conducted by the inventors will be described hereinafter.
FIGS. 39A and 39B are graphs for showing results of observing meniscus vibration by using laser Doppler velocimeter, wherein the volume velocity of the ink at the nozzle is plotted against the time. FIG. 39A showing a typical normal state conceived and FIG. 39B showing an abnormal state observed. More specifically, the meniscus vibration shown in FIG. 39A is normal and thus expected to be observed; however, as shown in FIG. 39B, the meniscus vibration actually observed had additional very small vibration superposed on the typical meniscus vibration. The small vibration superposed on the typical meniscus vibration causes much instability in the ejection of ink droplets. By the meniscus control technique described above, in particular, the liquid surface interference in the meniscus is utilized to eject very small droplets. If, therefore, such small vibration is superposed on the typical meniscus vibration, desired very small droplets cannot be ejected, or on the other hand, unwanted excess ink droplets may be ejected, whereby a normal ejection of small droplets cannot be expected any more.
In view of the above disadvantages in the conventional techniques, it is an object of the present invention to provide an inkjet recording head which is capable of allowing xe2x80x9clarge dropletsxe2x80x9d having desired diameters to be ejected from a single nozzle and achieving xe2x80x9cincrease in the nozzle densityxe2x80x9d to thereby improve the ink droplet ejection efficiency per unit area while preventing the head size and the cost from increasing.
It is also an object of the present invention to provide an inkjet recording apparatus including such an inkjet recording head, and to provide methods of manufacturing and driving such an inkjet recording head.
Another object of the present invention is to provide an inkjet recording head that allows both xe2x80x9clarge dropletsxe2x80x9d and xe2x80x9csmall dropletsxe2x80x9d each having a desired size to be selectively ejected from a single nozzle, thereby achieving both high speed recording and high image quality recording.
Yet another object of the invention is to provide an inkjet recording head having high ejection stability that can prevent abnormal vibration of a meniscus.
Thus, the present invention provides an inkjet recording head including a plurality of nozzles, a plurality of pressure chambers each disposed in communication with a corresponding one of the nozzles, at least one diaphragm disposed to form a part of the wall surfaces of the pressure chambers, and a plurality of piezoelectric actuators each disposed for a corresponding one of the pressure chambers in operative relationship with the diaphragm. A part of the diaphragm and each piezoelectric actuator in combination constitute a vibrating member which deforms to generate a pressure wave in the ink filled within the pressure chamber, thereby ejecting ink droplets from the nozzle. In the feature of the present invention, it is defined that acoustic capacitance of the vibrating member is set at 2.0xc3x9710xe2x88x9220 m5/N or higher.
In accordance with the inkjet recording head of the present invention, large droplets each having a desired size can be ejected from the same nozzle. In addition, increase in the nozzle density can be achieved to improve the ink ejection efficiency per unit area while preventing the head size and the cost of the inkjet recording head from increasing.
A method of manufacturing an inkjet recording head according to the present invention includes the step of forming a pattern for the piezoelectric actuators by sandblast processing.
By the method of manufacturing the inkjet recording head according to the present invention, the pattern of the piezoelectric actuators formed by the sandblast processing allows the piezoelectric actuators having a complicated shape to be suitable for maximizing the ejection efficiency, and also the electric connection for the piezoelectric actuators can be achieved with high dimensional precision and with less cost.
An inkjet recording apparatus according to the present invention includes the inkjet recording head as described above. Use of the inkjet recording apparatus according to the present invention allows both high speed recording and high image quality recording.
A first method of driving an inkjet recording head according to the present invention is directed to the inkjet recording head which includes a plurality of nozzles, a plurality of pressure chambers each in communication with a corresponding one of the nozzles, a diaphragm forming a part of the wall surfaces of the pressure chambers, and a plurality of piezoelectric actuators each coupled to the diaphragm. A part of the diaphragm (or one of the plurality of diaphragms) and each piezoelectric actuator in combination form a vibrating member, which is deformed to compress the ink filled within a corresponding pressure chamber, to eject an ink droplet from the nozzle. The acoustic capacitance of the vibrating member is set at 2.0xc3x971020 m5/N or higher.
In the first method of the present invention, a driving voltage is applied to the vibrating member, wherein the waveform of the driving voltage includes a first voltage change process to be applied in the direction to compress the volume of the pressure chamber and allow an ink droplet to be ejected, and a second voltage change process to be applied in the direction to expand the volume of the pressure chamber. Thus, an ink droplet of at least 15 pl is ejected.
According to the first method of driving an inkjet recording head according to the invention, large ink droplets necessary for low resolution recording 600 dpi or less can be ejected from the inkjet recording head driven by the method.
A second method of driving an inkjet recording head according to the present invention is directed to the inkjet recording head which includes a plurality of nozzles, a plurality of pressure chambers each in communication with a corresponding one of the nozzles, a diaphragm forming a part of the wall surface of the pressure chambers, and a plurality of piezoelectric actuators each coupled to the diaphragm. A part of the diaphragm or one of the plurality of diaphragms and each piezoelectric actuator in combination form a vibrating member. The vibrating member is deformed to compress the ink filled within the pressure chamber, to eject an ink droplet from the nozzle. The acoustic capacitance of the vibrating member is set in the range from 2.0xc3x9710xe2x88x9220 m5/N to 5.5xc3x9710xe2x88x9219 m5/N.
In the second method, a driving voltage is applied to the vibrating member, wherein the waveform of the driving voltage includes a first voltage change process to be applied in the direction to expand the volume of the pressure chamber, and a second voltage change process to be applied in the direction to compress the volume of the pressure chamber, to form a liquid column of ink having a diameter smaller than the nozzle size of the nozzle, and separate and eject a very small ink droplet from the tip of the liquid column. Thus, an ink droplet of 4 pl or more is ejected.
By the second method of driving an inkjet recording head according to the present invention, image recording with low granularity and high image quality can be achieved.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.